So far, most of flat panel displays are liquid crystal displays (LCD), however, people around the world have been trying with great efforts to develop a new flat panel display different from LCD, which is more economical and has an outstanding performance. Recently, organic electroluminescent devices, as the next generation of flat panel displays, have attracted much attention. Compared with LCDs, organic electroluminescent devices have many advantages, such as self-luminescence, wide angle of view, low driving voltage, fast response speed, potential of achieving flexible display lamps, etc. Since they were invented in 1980s, organic electroluminescent devices have already been applied in industrial practice, such as manufacturing of cameras, computers, mobile phones, TV displays, etc. Although the technology of organic electroluminescent device has been greatly developed over the years due to continuous investments and unremitting efforts from all over the field, it is still restricted by many problems, such as short life expectancy, low efficiency, etc.
An organic electroluminescent device includes a substrate, an anode, a hole injection layer for accepting holes from the anode, a hole transport layer for transporting holes, an emission layer where holes and electrons are combined to emit lights, an electron blocking layer for preventing electrons entering the hole transport layer from the emission layer, an hole blocking layer for preventing holes entering the electron transport layer from the emission layer, an electron injection layer for accepting electrons from cathode.
The driving mechanism of the organic electroluminescent device is described as follows: when voltage is applied between the anode and the cathode, holes injected from the anode travel via the hole injection layer and the hole transport layer into the emission layer. Meanwhile, electrons injected from the cathode travel via the electron injection layer and electron transport layer into the emission layer. The current carriers and electrons are recombined to generate excitons within the emission layer. Under the current status, the excitons change into the ground state, and accordingly, the fluorescent molecules in the emission layer emit lights to form images. Here, when excitons return to the ground state through a singlet excited state, the lights emitted are called fluorescence; when excitons return to the ground state through a triplet excited state, the lights emitted are called phosphorescence. The probability for excitons to transfer through a singlet excited state to the ground state is 25%, while the probability through a triplet excited state to the ground state is 75%. Therefore, for organic electroluminescent devices emitting fluorescence, the luminous efficiency is limited; however, for organic electroluminescent devices emitting phosphorescence, emissions can be caused by 75% of triplet excitons and 25% of singlet excitons, rendering the internal quantum efficiency up to 100% in theory. The phosphorescent emission layer includes main body material and dopant material. The dopant material accepts energy from the main body material to emit lights. The dopant material may include Iridium compounds, which yet may cause problems such as low luminous efficiency of blue lights and short life expectancy. It is of great urgency, with the enlargement of displays' sizes, to invent a new blue dopant material for solving the above problems.